Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus.
Description of the Related Art
A conventional electro-photographic image forming apparatus such as a printer and a copier includes a sheet feeding portion feeding a sheet stacked in a sheet stacking portion and a sheet conveying portion conveying the sheet fed from the sheet feeding portion to an image forming portion. Here in the image forming apparatus, it is essential to accurately match a toner image formed in the image forming portion with a front end of the sheet conveyed from the sheet feeding portion at a transfer portion.
However, there is a case when a sheet feed starting position shifts in the sheet feeding apparatus due to a slip of the sheet, a shift of a position of the sheet stacked on the sheet stacking portion, and a double feed caused by a preceding sheet. In such a case, the feed of the sheet varies and it becomes hard to accurately match the toner image with the front end of the sheet at the transfer portion.
Still further, some conventional image forming apparatuses include a sensor detecting a front end of a fed sheet. The image forming apparatus forms a toner image on a photosensitive drum based on timing when the sensor detects a front end position of the fed sheet, primarily transfers the toner image thus formed to an intermediate transfer body and then secondarily transfers the toner image to the sheet at a secondary transfer portion. In such image forming apparatus, there is a case when a moving distance of the toner image from the photosensitive drum to the secondary transfer portion is longer than a sheet conveying distance from the front end position of the sheet stacked on the sheet stacking portion to the secondary transfer portion. In this case, the sheet arrives at the secondary transfer portion before the toner image arrives at the secondary transfer portion if the toner image is started to be formed on the photosensitive drum after detecting the front end position of the sheet fed from the sheet feeding apparatus by the sensor.
To that end, the conventional image forming apparatus is provided with a registration portion at downstream of the sheet feeding apparatus to let the front end of the sheet temporarily stand by at the registration portion and to convey the sheet again by matching with a timing when the toner image arrives at the secondary transfer portion. As disclosed in Japanese Patent Laid-open No. Sho. 63-147739, such registration portion is provided with a shutter member to temporarily stop a sheet passing through a sheet conveying path by the shutter member. The image forming apparatus provided with such registration portion includes a torque control mechanism between a driving shaft of a conveying roller disposed upstream of the shutter member and the conveying roller. Then, the registration portion is configured such that while controlling the front end of the sheet by the shutter member, the conveying roller temporarily stops its rotation by idly rotating the driving shaft by an action of the torque control mechanism to let the sheet stand by temporarily.
While the conventional image forming apparatus provided with such registration portion stops the conveying roller by the torque control mechanism during when the sheet is stopped by the shutter member, a feed roller disposed upstream of the conveying roller continuously feeds a sheet. Due to that, when the sheet temporarily stands by, a loop of the sheet is formed between the conveying roller and the feed roller. Here, in the configuration in which the loop is formed between the conveying roller and the feed roller, it is necessary to assure an enough loop forming space in which the loop of the sheet can be formed between the conveying roller and the feed roller.
By the way, because image forming apparatuses are required to be downsized lately, the distance between the feed roller and the conveying roller is shortened. If the distance between the feed roller and the conveying roller is shortened, the loop forming space is narrowed. If a loop is to be formed in such narrow loop forming space, a height of the loop with respect to a sheet feeding direction increases, destabilizing the feed of sheets such that a thin sheet whose rigidity is low causes buckling and a thick sheet whose rigidity is high increases a feeding load.